Purification of hydrocarbons



Nov. 19, 1968 w c ETAL 3,412,171

PURIFICATION OF HYDROCARBONS Filed June 27, 1966 tuna-chm mwmzuozOumummommq United States Patent 3,412,171 PURIFICATION OF HYDROCARBONSLester M. Welch, Seabrook, and Lloyd D. Tschopp and Rudolph C. Woerner,Houston, Tex., assignors to Petro- Tex Chemical Corporation, Houston,Tex., a corporation of Delaware Filed June 27, 1966, Ser. No. 560,639 5Claims. (Cl. 260681.5)

ABSTRACT OF THE DISCLOSURE A process for the purification of unsaturatedhydrocarbons, particularly diolefins such as butadine-1,3 or isoprenefrom a gaseous mixture containing hydrocarbons including acetylenichydrocarbons and relatively noncondensable gases including oxygen. Gasesto be purified may be obtained by oxidative dehydrogenation.Purification by a particular process including the use of benzene ortoluene as an absorber oil.

Unsaturated hydrocarbons such as styrene, butene and butadiene arecommercially produced by the catalytic dehydrogenation of more saturatedhydrocarbons. Butadiene is produced in large quantities by thedehydrogenation of butane and butene. Improved processes whereby higherconversions, yields and selectivities of products are desired.Additional improvements in the processes are also desired.

Improved processes for the preparation of unsaturated hydrocarbons suchas butenes, butadiene-1,3, isoprene or styrene are processes wherebyhydrocarbons such as butane, butene, isopentene, isopentane orethylbenzene are dehydrogenated at elevated temperatures in the presenceof catalysts, oxygen, and suitably halogen. Superior results and yieldsof products are thereby obtained. However, the product streams containnot only the desired unsaturated hydrocarbons but also may containvarious by-products such as C0, C0 hydrogen, nitrogen, oxygen,oxygenated hydrocarbons, acetylenic compounds, unreacted hydrocarbons,etc. When air is used as a source of oxygen, the eflluent from thedehydrogenation reactor will contain large quantities of certainrelatively non-condensable gases, such as nitrogen. The gaseous efiiuentwill also contain varying amounts of steam.

Various problems exist in regard to the economic separation andpurification of unsaturated hydrocarbons produced by oxidativedehydrogenation which are not encountered in the recovery of productsproduced by dehydrogenation in the absence of oxygen. Consequently,techniques utilized for the recovery and purification of productsderived from the dehydrogenations in the absence of oxygen have notgenerally been found satisfactory for the recovery of effiuentsresulting from oxidative dehydrogenation reactions. The presence oflarge quantities of oxygen, by-products gases, and gases such asnitrogen create entirely ditferent problems from those previouslyencountered. One particular problem encountered is the problem in regardto the removal ofacyclic acetylenic compounds, e.g. when the desiredproduct is a mono-olefin or a diolefin. Many unsaturated hydrocarbonproducts, such as butadiene-1,3, have rather strict maximum requirementsof acetylenic compounds. The quantity of acetylenic compound shoulddesirably be reduced to a relatively minor mol percent of thehydrocarbon product. Furthermore, another problem encountered in therecovery of eflluents from oxidative dehydrogenation reactors is that ahigh degree of fouling of recovery equipment is encountered. In view ofthese and other problems, a process was needed which would recover andpurify the un- 3,412,171 Patented Nov. 19, 1968 saturated hydrocarbon inan economical and etficient manner.

According to this invention, a process has been discovered whereby theunsaturated product may be recovered from the various gases present andfrom the acetylenic compounds at the same time. According to thisinvention, a particular gaseous mixture comprising unsaturatedhydrocarbons, oxygen and inert non-condensable gases may be separated byintimately contacting the gaseous mixture with a composition comprisingbenzene, toluene or mixtures thereof. The contacting zone is preferablymaintained at a temperature of between 55 F. and 150 F. and a pressurebetween p.s.i.g and 200 p.s.i.g. The liquid composition from thecontacting zone is then separated, such as by stripping, to recover theunsaturated hydrocarbons.

The zone wherein the gaseous mixture is contacted with the benzene,toluene or mixtures thereof, may be any suitable equipment for absorbingthe gaseous mixture in the composition. This absorber may be e.g. acolumn having bubble cap trays or perforated plates or may be a packedcolumn or the like. The zone wherein the unsaturated hydrocarbon isstripped from the resulting liquid composition may be any equipment toperform this function. One method for stripping off the unsaturatedhydrocarbons is to feed the composition to the top or near the top of afractionating column such as a tray type or packed column. Although lessperferred it is also possible to flash off the gases in equipment sodesigned. Preferred equipment for the stripping zones are plate columns(perforated, value, bubble cap, etc.) and packed columns. Also it ispreferred to feed the composition to the top /3 of the stripping column.

The gaseous mixture to be treated containing the unsaturatedhydrocarbon, non-condensable gases, oxygen, by-product acetylenes andvarious other by-products may be obtained from a variety of sources.However, the invention is particularly suitable for the purification ofgaseous effluents resulting from the oxidative dehydrogenation ofhydrocarbons utilizing air or oxygen diluted with non-condensablediluents such as nitrogen or helium. Halogens may be added to increasethe yields and selectivities of the desired product. A preferred sourceof halogen is from ammonium halides as disclosed in U.S. 3,207,805.Examples of processes for dehydrogenation in the presence of oxygen arefound in U.S. Patents 3,207,805 through U.S. 3,207,811, also e.g. inExamples I of U.S. 3,159,688, U.S. 3,205,280 and according to U.S.3,080,435 wherein a molten salt reactor containing a metallic iodide isemployed.

Hydrocarbons to be dehydrogenated are acyclic, cycloaliphatic or alkylaryl hydrocarbons of 3 to 9 carbon atoms, which contain at least twoadjacent carbon atoms, each of which carbon atom has at least onehydrogen atom attached. The dehydrogenation will produce compoundshaving double and/or triple bonds. Thus, butadiene-1,3 and/ orvinylacetylene may be produced from butene-l or butene-2 or mixturesthereof, and isoprene may be produced from any of the methyl butenes,such as 2-methyl butene-l, 2-methyl butene-2 or 3-methyl butene-l ormixtures thereof. Isoprene may also be produced from methyl butanes,such as Z-methyl butane; also olefins and diolefins may be produced fromsaturated hydrocarbons, for example, vinyl acetylene, butadiene andbutene may be produced from n-butane. A mixture of mono-olefins anddiolefinsmay also be produced, such as a mixture of butadiene-1,3 andbutenes from a feedstock of a mixture of n-butane and butene.Cyclohexane may be dehydrogenated to cyclohexene and/or benzene. Ethylbenzene or ethylcyclohexane may be dehydrogenated to styrene.

Good results have been obtained with a feed containing at least 50, andpreferably at least 75, mol percent of an acyclic aliphatic hydrocarbon,such as the hydrocarbons of 4 to 5 carbon atoms having a straight chainof at least four carbon atoms and single double bond; preferred are themonoethylenically unsaturated compounds or mixtures of saturated andunsaturated compounds.

Oxygen will generally be supplied to the dehydrogenation zone in therange of about 0.20 mol of oxygen to 2.0 or 3.0 mols of oxygen per molof hydrocarbon to be dehydrogenated. A preferred range for the oxygen isfrom about 0.3 to 1.50 mols of oxygen per mol of hydrocarbon to bedehydrogenated. Either air or oxygen diluted with diluent such asnitrogen, helium and the like may be utilized. Steam may be fed to thedehydrogenation zone in amounts such as from about 2 to 40 mols of steamper mol of hydrocarbon to be dehydrogenated. An advantageous range isfrom 2 to 20 mols of steam per mol of hydrocarbon. If halogen isemployed, the halogen will suitably be present in an amount from about.001 to 0.1 mol per mol of hydrocarbon fed.

The dehydrogenation reaction may be conducted in the absence of contactcatalysts, but better results are obtained if the reaction is conductedin the presence of metal or metal compound catalysts. Thedehydrogenation reactor may be a fixed or fluid bed reactor. Reactorssuch as those conventionally used for the dehydrogenation ofhydrocarbons to butadiene may be employed. The total pressure in thedehydrogenation zone may suitably be about atmospheric pressure.However, higher pressures or vacuum may be used. Pressures such as fromabout atmospheric (or below) up to about 100 to 200 ps.i.g. may beemployed. The dehydrogenation reaction will normally be conducted at atemperature of reaction between about 600 F. to about 1500 F. or higheralthough generally the maximum temperature in the reactor will be withinthe range of about 700 F. and 1300 F. This temperature of the reactionis measured at the maximum temperature in the reactor. The flow rates ofthe reactants may be varied quite widely and will be dependent somewhaton whether fixed or fluid bed reactor is employed. Good results havebeen obtained with flow rates of the hydrocarbon to be dehydrogenatedranging from about A to 25 liquid volumes of hydrocarbon to bedehydrogenated per volume of reactor zone per hour, with the volumes ofhydrocarbon being calculated as the equivalent amount of liquidhydrocarbons at standard conditions of 156 C. and 760 millimeters ofmercury absolute. For the purpose of calculating flows, the reactionzone is defined as the portion of the reactor which contains catalystand which is at a temperature of at least 600 F. In other words, thevolume of the reaction zone is equivalent to the volume of the catalystzone if it were empty. The residence or contact time of the reactants inthe dehydrogenation zone depends on several factors involved in thereaction. Contact times such as about 0.001 to about 5, or 25 secondshave been found to give excellent results. Under certain conditions,higher contact times may be utilized. Contact time is the calculateddwell time of the reaction mixture in the reaction zone assuming themols of product mixture are equivalent to the mols of feed.

The efiluent from the dehydrogenation zone will contain the impureunsaturated hydrocarbon products, oxygen, various impurities includingoxygenated hydrocarbons, non-condensable inert gases and depending uponthe particular process, perhaps some unconverted feed or halogenatedcompounds. If air was used as the source of oxygen, nitrogen will bepresent in relatively large quantities as a non-condensable gas. Steammay be present in an amount up to 96 mol percent of the total efiluent,such as from about 5 to 96 mol percent. The organic phase includingdehydrogenated product, any unreacted feed, oxygenated hydrocarbons, anyhalogenated compounds, polymer and tar and precursors thereof and anyorganic decomposition products usually range from about 3 to 50 molpercent of the effluent and generally will be within the range of about3 to 30 or 35 mol percent of the efiiuent. The non-condensable gases(under the conditions encountered), such as nitrogen, will be present inan amount of from or about 20 to 93 mol percent of the total efliuent.

The effluent gases leaving the dehydrogenation zone will generally be attemperature of about or greater than 600 F. or 700 F. to 1600 F.depending upon the particular dehydrogenation process. The effluentgases are then cooled prior to further treatment according to thisinvention. The reactor efiluent may be cooled by any means orcombination of means as by quenching followed by employing waste heatboilers, condensers, vapor separators and the like. Ordinarily, waterwill be removed as condensed steam from the gaseous efiluent during-thiscooling operation. This cooled gaseous stream may then be treatedaccording to the present invention or may first be processed to removecarbonyl compounds or halogenated compounds such as by the process ofUS. 3,200,166.

A preferred embodiment of the invention is illustrated in the drawing.According to the process of the drawing, a separator is employed inaddition to the absorber and the hydrocarbon stripper. This sequencetogether with the process conditions is claimed in copending applicationSer. No. 560,638 filed on even date herewith. The gas feed 1 may beobtained from any suitable source, such as from the dehydrogenation ofhydrocarbons in the presence of oxygen to form a mixture of inertnon-condensable gases, unsaturated hydrocarbons, unreacted hydrocarbons,oxygenated hydrocarbons, acetylenes, oxygen, nitrogen, water and variousother by-products, such as CO and CO. The dehydrogenation reactoreffiuent gen erally will be cooled such as by quenching and by indirectheat exchange prior to entering the absorber. Also, some of the steammay be removed by means such as knockout vessels and the like. Anyhalogens or halogen compounds Will preferably be removed upstream. Thegaseous feed 1 will comprise or consist, exclusive of any water present,from 3.5 to mol percent unsaturated hydrocarbon, from .001 to 3 molpercent oxygen, from 20 to 93 mol percent inert non-condensable gases(this term refers to non-hydrocarbons such as H N CO CO, helium, and thelike which are not condensable under the conditions encountered). Thegaseous feed 1 may also optionally contain from 0.003 to 7 mols ofwater, either as steam or as entrained water, per mol of totalhydrocarbon. Based on the total organic content of the gaseous feed 1,the total hydrocarbons will constitute at least mol percent of theorganic portion of this gaseous feed 1. Preferably, the composition ofthe gaseous feed 1, exclusive of any water present, will be from 5 to 65mol percent unsaturated hydrocarbons, from .001 or .01 to 1.0(preferably less than 0.3) mol percent oxygen, from 45 to 89 mol percentinert non-condensable gases, and the total hydrocarbons will constituteat least mol percent of the organic portion of the gaseous feed 1. Also,preferably, water will be present as steam in an amount of from 0.003 to10 mols of steam per mol of total hydrocarbon in the gaseous feed.

Lean oil 2 comprises benzene, toluene or mixtures thereof and willpreferably be fed to the top of the absorber in order to havecountercurrent contact with the gaseous feed 1 which is rising in thetower. The lean oil will predominately consist of benzene or toluene ormixtures thereof. Of course, the lean oil may contain impurities,particularly after the process has been in operation for a period oftime.

The absorber is suitably operated within a temperature of from 60 F. to150 F. and, more desirably, within the range of 80 F to 135 F. Thepressure in the absorber will befrom ps.i.g. to 200 ps.i.g. and, moredesirably, from ps.i.g. to ps.i.g. According to this preferredembodiment, the absorber does not contain a reboiler. That is, theabsorber is not a fractionating absorber.

During operation of the process, some impurities will be encountered inthe recycling lean oil. Nevertheless, the lean oil 2 entering the top ofthe absorber should have a composition containing predominately, andpreferably, at least 65 mol percent of benzene, toluene or mixturesthereof. Means may be provided to purify the lean oil to remove heaviermaterials, such as by distillation, p r to recirculating the lean oil tothe absorber.

Suitably, coolers such as 3 and 4 may be inserted into the absorbersystem in order to maintain the required reaction conditions in theabsorber. Also, not shown, the absorber may have incorporated a spongeoil unit to recover lean oil going overhead from the absorber. This leanoil coming overhead may be purified, such as by absorption and strippingin the sponge oil unit and returned to the lean oil system at any point.

The liquid composition 5 leaving the absorber comprises the fat oilcontaining absorbed gases. This composition may then be cooled in cooler6. Any suitable means for cooling this composition may be utilized, suchas a heat exchanger cooled by refrigerant or cooled water.

The cooled composition 7 is then fed to the separator. The composition 7is preferably fed to the upper onethird of the separator, and,preferably, is fed to the top tray of the separator. Heat is added tothe separator, such as by a reboiler 8. In the separator, the largequantities of inert noncondensable gases inCludin-g oxygen, nitrogen, COand various 0 's and C s, may be taken 01f overhead together withacetylene compounds. Exclusive of any water present, in the separator,preferably at least 1 mole percent of the stream 7 is removed as anoverhead gaseous composition 9. The gaseous overhead 9 from theseparator may then be disposed of in any manner. The gaseous composition9 may be returned to the inlet for the compressors compressing thereactor efliuent or may be fed to separate compressors and thereaftermay be recycled to the gaseous feed 1 entering the absorber. The gaseousoverhead 9 may also be cooled and collected in an accumulator (notshown) from which the composition is recycled to the separator and thegaseous overhead from the accumulator may then be sent to a compressorand thereafter fed to the gaseous feed line 1 or utilized otherwise.According to this scheme, excellent removal of acetylenic compounds isachieved. The liquid composition 10 is fed to the hydrocarbon stripperwherein the unsaturated hydrocarbon is stripped from the lean oil andtaken off overhead as 11. The lean oil 12 is taken off from the stripperand may be purified by means not shown prior to returning to theabsorber as 2. The hydrocarbon stripper will have means for heating,such as by the reboiler 13. The unsaturated hydrocarbon 11 comingoverhead may then be sent for further purification, for example, toseparate the unsaturated hydrocarbon from the remaining hydrocarbons.

Tlhe separator may suitably be operated at a temperature of betweenabout 55 F. and 190 F. and a pressure of between about -5 p.s.i.g. and70 p.s.i.g., with the preferred temperature range being from 70 F. to160 F. and the preferred pressure range being between 15 p.s.i.g. and 50p.s.i.g.

The invention will be illustrated for the purification of-butadiene-1,3. Butadiene is obtained by oxidative dehydrogenation. Theeffluent from the reactor is cooled and steam is removed throughknock-out vessels. The resulting gaseous stream is then processedaccording to this invention, with reference being made to the drawing.The gaseous feed 1 contains 18 mol percent butadiene, 12

' mol percent total butene and butane, 0.1 mol percent oxygen, 69.9 molpercent inert noncondensable gases (including H N CO CO and helium). Thegaseous feed 1 also contains 0.6 mols of water per 100 moles feed.

Lean oil 2 is fed to the top of the absorber and the gaseous feed 1 isfed to the bottom of the absorber. The

lean oil 2 is benzene with a boiling point of 176 F.

The absorber is operated with a bottoms temperature of about 130 F. inan overhead temperature of 70 F. The pressure in the absorber is aboutp.s.i.g. The absorber does not contain a reboiler. The absorber isequipped with a sponge oil unit to recover lean oil going overhead fromthe absorber.

The liquid composition 5 leaving the absorber comprises the fat oilcontaining absorbed gases. This composition is cooled in cooler 6 and istransmitted to the top of the separator. Heat is added to the separatorby reboiler 8. The separator is operated at a bottoms temperature of F.and an overhead temperature of about 85 F. In the separator,methylacetylene, together with large quantities of inert, noncondensablegases are taken off overhead. In the separator 3.0 mol percent of thestream is removed as an overhead gaseous composition 9. The liquidcomposition 10 is fed to the hydrocarbon stripper wherein unsaturatedhydrocarbons are stripped from the lean oil and taken off overhead as11. The hydrocarbon stripper bottoms temperature is about'250 F. Thelean oil 12 is purified by means not shown and returned to the absorberas lean oil 2. The unsaturated hydrocarbon 11 coming overhead from thehydrocarbon stripper is then further purified to produce butadiene-1,3of a composition of at least 99.1 mole percent purity wherein themethylacetylene is present in an amount of less than 0.01 mol percent.

When this example is repeated utilizing a gaseous stream 1 wherein themajor hydrocarbon component is isoprene instead of butadiene-1,3 theadvantages'of the invention are also realized.

We claim:

1. A process for the preparation of unsaturated hydrocarbons comprisinga member selected from the group consisting of butadiene-1,3, isopreneor mixtures thereof without the formation of excessive polymer whichcomprises (1) oxidatively dehydrogenating hydrocarbons contaming atleast 50 mol percent acyclic aliphatic hydrocarbons of 4 to 5 carbonatoms having a straight chain of at least four carbon atoms with saidhydrocarbons of 4 to 5 carbon atoms having at least two adjacent car-bonatoms each of which has at leastone carbon atom attached to produce agaseous mixture comprising exclusive of any water present from 3.5 to 80mol percent unsaturated hydrocarbons including the said butadiene-1,3,isoprene or mixtures thereof, and from 20 to 93 mol percent inertnoncondensable gases containing from .001 to 3 mol percent oxygen,

(2) intimately contacting in an absorbing zone the said gaseous mixturewith an oil consisting essentially of toluene,

(3) taking off from the said absorbing zone a liquid compositioncomprising the said oil and dissolved hydrocarbons,

(4) separating the said liquid composition of (3) by stripping with heatto remove dissolved lhydrocarbons as a volatile fraction and to producea lean oil containing dissolved therein material heavier than toluene,

(5) removing the said heavier materials from (4) by distillation fromthe toluene,

(6) feeding the purified oil consisting essentially of toluene from (5)to the said absorbing zone of (2) as absorber oil.

2. A process for the preparation of butadine-1,3 without the formationof excesive polymer which comprises (1) oxidatively dehydrogenatinghydrocarbons containing at least 50 mol percent butene to produce agaseous mixture comprising exclusive of any water present from 3.5 to 80mol percent unsaturated hydrocarbons, and from 20 to 93 mol percentinert noncondensable gases including oxygen in an amount of from .001 to1.0 mol percent oxygen,

(2) intimately contacting in an absorbing column the said gaseousmixture with an oil consisting essentially of toluene, the saidabsorbing column being operated within the range of 80 F. to 135 F. andat a pressure of from 100 p.s.i.g. to 200 p.s.i.g.,

(3) taking off from the said absorbing zone a liquid compositioncomprising the said toluene and dissolved hydrocarbons,

(4) separating the said liquid composition of (3) by feeding the liquidcomposition to the top one-third of a stripping column and strippingwith heat to remove dissolved hydrocarbons as a volatile fraction and toproduce a lean oil containing dissolved therein material heavier thantoluene,

(5) removing the said heavier materials from (4) by distillation andtaking off toluene in a volatile fraction,

(6) condensing toluene from (5) and feeding condensed toluene to thesaid absorbing zone of (2) as absorber oil.

3. A process for the preparation of butadiene-1,3 containing less than0.01 mol percent methyl acetylene without the formation of excessivepolymer which comprises (1) oxidatively dehydrogenating hydrocarbonscontaining at least 50 mol percent butene to produce a gaseous mixturecomprising exclusive of any water present from 3.5 to 80 mol percentunsaturated hydrocarbons including butadiene-l,3 and a minor amount ofmethyl acetylene, and from to 93 mol percent inert noncondensable gasescontaining oxygen in an amount of less than 0.3 mol percent oxygen,

(2) intimately contacting the said gaseous mixture in an absorbingcolumn with an oil consisting essentially of toluene, the said absorbingcolumn being maintained at a temperature of between about 60 F. and 150F. and a pressure of between 100 p.s.i.g. and 200 p.s.i.g.,

(3) taking off from said absorbing column a liquid compositioncontaining the said toluene and butadiene-l,3 absorbed therein,

(4) cooling the said liquid composition of (3) to a temperature of nogreater than 100 R,

(5) feeding the cooled product from (4) to a column which is maintainedat a temperature of between F. and 190 F. and a pressure between 5p.s.i.g. and p.s.i.g.,

(6) heating the said column of (5) to take off a gaseous mixturecomprising oxygen, inert noncondensable gases and methyl acetylene,

(7) taking off from the said column of (5) a liquid compositioncomprising the said toluene and butadiene-1,3 dissolved therein,

(8) separating the said liquid composition of (7 by stripping with heatto remove dissolved butadie'ne- 1,3 as a volatile fraction and toproduce a lean toluene containing dissolved therein material heavierthan toluene,

(9) removing the said heavier materials from (8) by distillation andtaking off toluene in a volatile fraction,

(10) condensing toluene from (9) and returning condensed toluene to thesaid absorbing column of (2) as absorber oil.

4. The method of claim 1 wherein the said unsaturated hydrocarbon isbutadiene-1,3.

5. The method of claim 1 wherein the said absorbing in step (2) isconducted at a temperature within the range of F. to 135 F. and at apressure of from p.s.i.g. to 200 p.s.i.g.

References Cited UNITED STATES PATENTS 2,573,341 10/1951 Kniel 260-6772,814,359 11/1957 Koble 25551 2,905,732 9/1959 Fauske 55-51 2,909,57910/1959 Schmidt et al. 260- 677 3,023,843 3/1962 Grubb et a1. 55643,235,471 2/1966 Clay 260681.5

FOREIGN PATENTS 956,048 4/1964 Great Britain.

DELBERT E. GANTZ, Primary Examiner.

HERBERT LEVINE, Assistant Examiner.

